Athletic Bilbao coach Ernesto Valverde said while his side could have won by a greater margin against Valencia, he was happy with the win in "dangerous" conditions.
Bilbao boosted their hopes of qualifying for the Europa League quarter-finals thanks to a 1-0 victory in torrential rain at San Mames on Thursday.
Ground staff attempted to clear the pitch before kick-off following the downpour, but both sets of players still struggled, though Raul Garcia popped up in the 20th minute to give Bilbao a slender advantage in the last-16 tie.
Speaking afterwards, Valverde said: "We could have score more goals, but Valencia also had chances to score.
"We have not conceded at home, and they have to win there [at Mestalla in the second leg]."
Of the conditions, Valverde continued: "It was raining for some time and the field was not able to absorb all the water.
"The field was slow and dangerous. For Sunday [against Real Betis] we must make use of all equipment. We need to retrieve the field."
Goalscorer Raul Garcia added via BeIN Sports: "We deserved a positive result. I'm happy to score but especially if it helps us win the tie.
"There were areas where you could not play, we had to adapt. It was a very important game and it went well."